The invention relates to a semiconductor laser device, and more particularly to a stained layer InGaAs quantum well semiconductor laser device formed on a GaAs substrate.
Recently, the strained layer InGaAs quantum well semiconductor laser on a GaAs substrate has been receiving a great interest for application to a long wavelength laser device. Importance in research and development for improvement of the strained layer InGaAs quantum well semiconductor laser will be on the increase. Typically, the strained layer InGaAs quantum well semiconductor laser has a double heterostructure comprising an InGaAs active layer and potential barrier layers sandwiching the active layer.
One of the most important factors for determining a possible high performance of the laser device is a quality of an interface between the InGaAs active layer and a potential barrier layer formed on the InGaAs active layer. Lattice defect free interface including completely no crystal defect nor lattice defect would be desired for obtaining an ideal property of the laser device. Another important factor for determining the device performance would aloe be a quality of the active layer. Introduction of the strained layer for the active layer serving as the quantum well is preferable to suppress increase of the defects within the active layer and on the interface between the active layer and the upper potential barrier layer formed on the active layer.
The strained layer may also contribute a reduction, due to a compressive stress, of a threshold current at or over which a laser emission may be obtained. The reduction of the threshold current is still another most important factor for the laser device. Since InGaAs for the active layer serving as the quantum well has a larger lattice constant than a lattice constant of the GaAs substrate, the InGaAs active layer formed on the GaAs substrate would be strained. That is why the InGaAs active layer is introduced in the GaAs substrate laser device.
Some types of the strained layer InGaAs quantum well semiconductor laser on the GaAs substrate have already been known in the art, typical one of which is disclosed in April 1993 Applied Physics Letters vol. 62, 16 1869-1871. A strained layer InGaAs quantum well laser is formed on a (100) GaAs substrate by use of organometallic vapor phase epitaxy in an atmospheric-pressure. This laser device falls into one of the typical laser devices wherein the substrate has a (100) plane with zero off angle.
This laser device formed on the (100) GaAs substrate will be described with reference to FIG. 1. An n-type (100) GaAs substrate 31 is prepared on which an n-type GaAs buffer layer 32 having a thickness of 0.1 micrometers is formed. An n-type Al.sub.0.6 Ga.sub.0.4 As cladding layer 33 having a thickness of 2.5 micrometers is grown on the n-type GaAs buffer layer 32. An Al.sub.0.35 Ga.sub.0.65 As optical guide layer 34 having a thickness of 0.1 micrometers is grown on the Al.sub.0.6 Ga.sub.0.4 As cladding layer 33. A GaAs potential barrier layer 35 having a thickness of 6 nanometers is grown on the Al.sub.0.35 Ga.sub.0.65 As optical guide layer 34. An In.sub.0.2 Ga.sub.0.8 As strained quantum well active layer 36 having a thickness of 6 nanometers is grown on the GaAs potential barrier layer 35. A GaAs potential barrier layer 37 having a thickness of 6 nanometers is grown on the In.sub.0.2 Ga.sub.0.8 As strained quantum well active layer 36. An Al.sub.0.35 Ga.sub.0.65 As optical guide layer 38 having a thickness of 0.1 micrometers is grown on the GaAs potential barrier layer 37. A p-type Al.sub.0.6 Ga.sub.0.4 As cladding layer 39 having a thickness of 2.5 micrometers is grown on the Al.sub.0.35 Ga.sub.0.65 As optical guide layer 38. A p-type GaAs cap layer 40 having a thickness of 0.25 micrometers is grown on the p-type Al.sub.0.6 Ga.sub.0.4 As cladding layer 39 to thereby form a wafer including the double heterostructure for subsequent formation of a ridge waveguide, resulting in a formation of the strained layer quantum well semiconductor laser.
The laser obtained shows a laser emission at a wavelength of 0.98 micrometers. The laser device was subjected to a constant current aging test at an initial output power of 100 mW and a temperature of 25.degree. C. to thereby confirm that the laser device has a performance stability at 4.times.10.sup.-6 /h of a degradation rate equivalent to an average operating current.
The above type of the laser device wherein the double heterostructure is formed on the (100) plane of the GaAs substrate has a good flatness of an interface between the strained layer InGaAs quantum well and the GaAs potential barrier layer overlying the quantum well layer because a growing surface of the wafer has only a step of a monoatomic layer, but faces to a serious problem with lattice defect. Particularly, the double heterostructure grown on the (100) plane of the GaAs allows the InGaAs strained active layer and interface between the active layer and the potential barrier layer overlying the active layer to include vacant lattice alignments. It is difficult to obtain a reliability of the laser device against a long time current flow.
Another one of the typical strained layer InGaAs quantum well laser is disclosed in 1991 EEE Photonics Technology Letters vol. 3, No. 5 pp. 406-408. A strained layer InGaAs quantum well laser is formed on a (100) off plane of the GaAs substrate by use of organometallic vapor phase epitaxy in an atmospheric-pressure. This laser device falls into another one of the typical laser devices wherein the substrate has a (100) off plane.
This laser device formed on a GaAs substrate having a 2.degree. off (100) plane toward (011) direction will be described with reference to FIG. 2. An n-type 2.degree. off (100) GaAs substrate 41 is prepared on which an n-type GaAs buffer layer 42 having a thickness of 0.5 micrometers is formed. An n-type Al.sub.0.4 Ga.sub.0.6 As cladding layer 43 having a thickness of 1 micrometer is grown on the n-type GaAs buffer layer 42. An Al.sub.x Ga.sub.1-x As graded-index optical confinement layer 44 having a thickness of 0.15 micrometers is grown on the Al.sub.0.4 Ga.sub.0.6 As cladding layer 43. An active region 45 is grown on the Al.sub.x Ga.sub.l-x As graded-index optical confinement layer 44. The active region 45 comprises alternative laminations of four GaAs potential barrier layers having a thickness of 12 nanometers and three In.sub.0.2 Ga.sub.0.8 As strained layer quantum wells having a thickness of 7 nanometers. An Al.sub.x Ga.sub.l-x As graded-index optical confinement layer 46 having a thickness of 0.15 micrometers is grown on the active region 45. A p-type Al.sub.0.4 Ga.sub.0.6 As cladding layer 47 having a thickness of 1 micrometer is grown on the Al.sub.x Ga.sub.l-x As graded-index optical confinement layer 46. A p-type GaAs cap layer 48 is grown on the p-type Al.sub.0.4 Ga.sub.0.6 As cladding layer 47 to thereby form a wafer including the double heterostructure for subsequent formation of a ridge waveguide, resulting in a formation of the strained Layer quantum well semiconductor laser. The laser obtained shows a laser emission at a wavelength of 0.98 micrometers.
The above type of the laser device wherein the double heterostructure is formed on the 2.degree. off (100) plane of the GaAs substrate has a problem with a high threshold current and also a wide full width at half maximum of photoluminescence as well as problem with shifting a photoluminescence peak wavelength toward a short wavelength.